Metal/Organic Interfaces

Transcription

1 Metal/Organic Interfaces from first-principles Georg Heimel Institut für f r Physik, Humboldt-Universit Universität t zu Berlin Dresden,

2 Outline Investigated Systems / Motivation Methods Overview over recent and current research Self-assembled monolayers on metals Metal/molecule charge-transfer systems Stretching/breaking single-molecule junctions Doping in single-molecule junctions Outlook Summary and Conclusions

3 The Early Days... Graz, Austria Institute of Solid State Physics Graz University of Technology Research Topics: P3P P4P P6P Raman Shift [cm ] X-ray and Raman under high pressure Bandstructure and Quantum Chemistry T=295 K Exp. Calc. Exp. Calc Optical Density 2P Photoluminescence Raman Intensity [arb. u.] Structural, Electronic, and Optical Material Properties of Organic Semiconductors (Experiment and Theory) Transition Energy [ev] Optical Spectroscopy

4 Hybrid Organic/Inorganic Devices Organic Electronics: Interface Properties: - Charge-injection barriers - Morphology control - Charge-carrier traps -... Lighting OSRAM, Display available from SONY Bulk Properties organic source dielectric drain Photovoltaics Fraunhofer ISE Freiburg, RFID and FET Holst Center, gate + -

5 DFT studies of SAMs on Metals head group: -NH 2, -CN, -NO 2, -F, -OH, etc. HS D. Riposan et al., J. Phys. Chem. B 110, (2006). CH 3 SH P.Cyganic et al., J. Phys. Chem. B 108, 4989 (2004). metal (111): Au, Ag, Cu, Pt 47 nm π-core: phenyls ethylenes ethinylenes acenes spacer: alkanes docking group: -SH -CN -NC -pyridine

6 Interest in and Application of SAMs Hydrophopic/philic surfaces Nanolithography Chemical sensing Organic electronics Molecular electronics... Corrosion protection Biocompatible Surfaces Functional surfaces 100 nm J. Park et al., Nature 417, 722 (2002). H. Akkerman et al., Nature 441, 69 (2006).

7 Methodology DFT bandstructure VASP, SIESTA PAWs, norm-conserving Troullier-Martins Pseudo-Potentials Plane-waves or LCAO basis set Standard GGA functionals (PW91, PBE) Quantum chemistry Gaussian03 Implementation and Coding (SIESTA) Surface Green s functions and transport COOP, fragment orbitals,... Data analysis & electrostatic models Mathematica, Fortran90, shell-scripts

8 Establish Links to Experiment Adsorption/Desorption energies STM images CH 3 b ε B Structure SH Core-level shifts A 2nm Infrared spectra dµ/dq 2 [arb. u.] Molecule SAM calc. S 2p 3/2 Energy [ev] W. Azzam et al., Langmuir 19, 4958 (2003). K. Heister et al., J. Phys. Chem. B 105, 6888 (2001). calc. exp exp. S 2p 3/2 Energy [ev] θ Wavenumber [cm -1 ] G. Heimel et al., Surf. Sci. 600, 4548 (2006) Number of -CH 2 - units

9 Workfunction and Level Alignment metal molecule metal molecule LUMO Φ Φ Energy Energy Φ E F E F HOMO

10 Charge-Injection Barriers metal interface organic Energy E F e V + e V HOMO j exp e V k T B

11 Interfaces in Molecular Electronics Au CN S NC S Au metal interface organic interface I metal Energy E F E F V HOMO

12 Outline Investigated Systems / Motivation Methods Overview over recent and current research Self-assembled monolayers on metals Metal/molecule charge-transfer systems Stretching/breaking single-molecule junctions Doping in single-molecule junctions Outlook Summary and Conclusions

13 The SAM as a Dipole Layer E V vac IP HS CN HS NH 2 EA µ µ LUMO HOMO left V vac left EA left IP Phys. Rev. Lett. 96, (2006). Phys. Rev. B 77, (2008). Acc. Chem. Res. 41, 721 (2008). Surf. Interface Anal. 40, 371 (2008). Adv. Func. Mater. 18, 3999 (2008). X X X X X X X right V vac CN right EA right IP NH 2 Surf. Sci. 600, 4548 (2006). Nano Lett. 7, 932 (2007). Langmuir 24, 474 (2008).

14 Interfacial Potential-Energy Steps 2 ρ(x,z) [-qe /Å2 ] Energy [ev] Surf. Interface Anal. 40, 371 (2008). Acc. Chem. Res. 41, 721 (2008). Langmuir 24, 474 (2008). PRB 77, (2008). Nano Lett. 7, 932 (2007). Surf. Sci. 600, 4548 (2006). PRL 96, (2006). E F 0-5 Energy [ev] Au HOMO HS CN -10

15 The Final Picture HS CN HS NH 2 work function level alignment CN X X Φ Surf. Sci. 600, 4548 (2006). Nano Lett. 7, 932 (2007). Langmuir 24, 474 (2008). Phys. Rev. Lett. 96, (2006). Phys. Rev. B 77, (2008). Acc. Chem. Res. 41, 721 (2008). Surf. Interface Anal. 40, 371 (2008). Adv. Func. Mater. 18, 3999 (2008). Φ Au E F X LUMO X X HOMO X NH 2

16 Workfunction and Level Alignment Au(111) HS Au(111) HS Energy [ev] Φ calc = -1.5 ev z-distance [Å] E vac Φ E F G. Heimel et al., Phys. Rev. Lett. 96, , (2006). G. Heimel et al., Nano Lett. 7, 932 (2007). G. Heimel et al., Acc. Chem. Res. 41, 721 (2008). PDOS [arb. u.] = HOMO =E 0 F Energy [ev] = HOMO-1 = HOMO L. Romaner et al., Small 2, 1468 (2006). G. Rangger et al., Surf. Interface Anal. 40, 371 (2008). L. Romaner et al., Phys. Rev. Lett. 99, (2007).

17 F4TCNQ on Coinage Metals NC F F CN Experiment (XSW) DFT NC F F CN F N Å 2.1 Å Au(111) 10 nm Cu(111) F. Jäckel et al., Phys. Rev. Lett. 100, (2008). L. Romaner et al., Phys. Rev. Lett. 99, (2007). G. Rangger et al., submitted.

18 Charge Flow vs. Orbital Occupation ( q e = q ecombined q eslab + q emonolayer ) electron accumulation electron depletion Reminiscent of CO on Ni(100): σ Æ metal metal Æ π* N. Koch et al., Phys. Rev. Lett. 95, (2005). L. Romaner et al., Phys. Rev. Lett. 99, (2007). L. Romaner et al., Small 2, 1468 (2006). R. Hoffmann, Rev. Mod. Phys. 60, 601 (1988).

19 Outline Investigated Systems / Motivation Methods Overview over recent and current research Self-assembled monolayers on metals Metal/molecule charge-transfer systems Stretching/breaking single molecule junctions Doping in single-molecule junctions Outlook Summary and Conclusions

20 Stretching Molecular Junctions I

21 Stretching Molecular Junctions II F F

22 Orbital Occupation and Stretching E F E F orbital occupation 2 0 orbital number molecule remains charge-neutral through stretching process! E L. Romaner et al., Small 2, 1468 (2006). L. Romaner et al., Phys. Rev. Lett. 99, (2007).

23 DFT-based Ballistic Transport PDOS [ev -1 ] HOMO LUMO LUMO+1 Current [ma] =E F 1 2 Energy [ev] Voltage [V] Transmission di/dv [µav -1 ] Energy [ev] Voltage [V]

24 Outline Investigated Systems / Motivation Methods Overview over recent and current research Self-assembled monolayers on metals Metal/molecule charge-transfer systems Stretching/breaking single-molecule junctions Doping in single-molecule junctions Outlook Summary and Conclusions

25 Summary E F Φ Au(111) U vac + BD + EA left IP left LUMO = HOMO E F Φ Φ mod Au EA SAM LUMO IP SAM HOMO Interfaces in self-assembled monolayers Level-alignment and work function Metal/molecule charge-transfer systems Molecular distortion and (back)donation Fermi-level pinning in molecular junctions Break junctions and (neutral) radicals

26 Acknowledgements X-ray Roland Resel (TUGraz) Martin Oehzelt (JKU Linz) Andreas Aichholzer (?) Felix Porsch (HasyLab) Manfred Kriechbaum (ÖAW) Michael Winokur (Madison, WI) Michael Hanfland (ESRF) Raman M. Chandrasekhar (Columbia, MO) Chris Martin (?) Suchi Guha (Columbia, MO) Wilhelm Graupner (AVL Graz) Dieter Somitsch (?) Peter Knoll (KFU Graz) Bandstructure Peter Puschnig (MU Leoben) Kerstin Hummer (Uni Wien) C. Ambrosch-Draxl (MU Leoben) UV/VIS Maria Daghofer (Oak Ridge) Surfaces & Clusters Lorenz Romaner (MU Leoben) Egbert Zojer (TUGraz) Mathis Gruber (FHI Berlin) Gerold Rangger (TUGraz) Peter Pacher (TUGraz) Jean-Luc Brédas (GaTech) Georg Kresse (Uni Wien) Chris Zangmeister (NIST) Roger D. Van Zee (NIST) Organics N. Koch (HUB) S. Duhm (Chiba) I. Salzmann (HUB) J. P. Rabe (HUB) B. Bröker (HUB) A. Vollmer (Bessy) F. Schreiber (TÜB) A. Gerlach (TÜB)